The same reason we need them for any other set of reasons: To do full-time work investigating different angles of current events and to provide, often, a less-biased report on things.
Of course, eliminating PR =/= eliminating journalists, so this comment is off the tracks anyway.
This isn't what a journalist does at all though and I think they're pretty honest about this if asked.
At best they take the facts and induce bias to provide them in a way that is most palatable to their audience and mirrors the belief of their audience to get more views and therefore more ad impressions. At worst they see themselves as a class above normal people and their job is to ensure the people below them think the right way.
Essentially in a world where the actual events is transmitted via video over social media moments after they occur, seems pretty redundant that we need it filtered through biased individuals who's business only existed because newspapers used to be the most efficient way of doing that.
That's a hugely broad question. Many are.